Polyimide resins are excellent in heat resistance, electrical insulation properties, and chemical resistance, have good mechanical properties, and thus are used in electrical/electronic applications. For example, polyimide resins are used as an insulating film or a protective coating agent on a semiconductor device; and a surface protection material or a base resin for a flexible circuit board, an integrated circuit, or the like; and furthermore, they are used in a case where an interlayer insulation film or a protective film of fine circuits are formed. In particular, in the case where a polyimide resin is used as a coating material, a protective material in which a formed body such as a polyimide film is bonded using an adhesive, a liquid polyimide resin solution, and the like have been used.
Polyimide resin solutions are broadly divided into two types of solution. One type of solution is a polyamic acid solution, which is a precursor of a polyimide resin. The other type of solution is a solution of a polyimide resin which is soluble in an organic solvent. However, the polyamic acid solution and polyimide resin solution are polymer solutions of a high-molecular-weight product, which has a high molecular weight and low solubility, and thus it is impossible to adjust the concentration of a solute to be high. Accordingly, for example, when a coating film is formed, it is necessary to volatilize a large amount of solvent, resulting in a problem of low productivity. Furthermore, in the case where a precursor solution of a polyimide resin is formed into a coating film, it is necessary to perform imidization, and the imidization must be performed at a heating temperature higher than 300° C. Therefore, for example, when such a polyimide resin precursor solution is used as a protective agent of a flexible substrate or the like, an adhesive of a formed body, or the like, a problem that a wiring material cannot withstand high temperatures, or the like occurs. Accordingly, a resin that can be cured at low temperatures (250° C. or lower), at which degradation of wiring does not occur, has been desired.
Regarding techniques of these polyimide resin solutions, a polyimide precursor solution in which a tetracarboxylic acid or a diester acid derivative thereof, and a diamine are dissolved and which has a high concentration and a low viscosity has been proposed (refer to, for example, Patent Documents 1 to 4).
A polyimide precursor solution in which a carboxylic acid having an amide bond in its structure and a diamine are dissolved and which has a high concentration and a low viscosity has also been proposed (refer to, for example, Patent Documents 5 to 7).
Furthermore, a photosensitive resin composition or a plasma etching resist containing a terminally half-esterified imide siloxane oligomer has been proposed (refer to, for example, Patent Documents 8 to 11).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-209609    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-217502    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-319389    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-319391    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2001-31764    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2001-163974    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2000-234023    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2000-212446    Patent Document 9: Japanese Unexamined Patent Application Publication No. 2001-89656    Patent Document 10: Japanese Unexamined Patent Application Publication No. 2001-125273    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2001-215702